1. Field of the Invention
The present invention relates to a method of producing a polyacrylic acid (salt)-based water-absorbent resin. Specifically, the present invention relates to a method of producing a water-absorbent resin with high physical properties and excellent safety at high productivity.
2. Description of the Related Art
In recent years, water-absorbent resins are developed as cross-linked synthetic polymers having a high degree of water absorbency, and are widely used for absorbent articles such as disposable diapers and sanitary napkins, water-retaining agents for agriculture and horticulture, and industrial sealing materials. As for the water-absorbent resins described above, many kinds of monomers and hydrophilic polymers are proposed as their raw materials. Among those materials, acrylic acid-based water-absorbent resins, which use as the monomer acrylic acid and/or salts thereof are most widely industrially used owing to their high water absorbency.
In particular, sanitary materials such as disposable diapers have achieved advanced functionality and have been thinned. The amount or ratio (ratio in an absorbing article, % by weight) of a water-absorbent resin to be used in the sanitary materials is increased to obtain the thinner sanitary materials, while securing the increase in the amount of absorbed water and leakage prevention ability. Therefore, the water-absorbent resins are required to have high physical properties.
Examples of the physical properties required for the water-absorbent resin include reduction in an amount of an uncross-linked water-soluble polymer, which is conflicting to an increase in absorption capacity (U.S. Pat. No. 4,654,039), and the physical properties under load such as absorption capacity under load and liquid permeability under load (U.S. Pat. No. 5,562,646).
The water-absorbent resin has a cross-linked structure preferably inside of polymer, more preferably on a surface of the polymer as well as the inside thereof, so the cross-linking method greatly affects the physical properties. To this end, there have been proposed methods involving using particular cross-linking agents or particular cross-linking conditions in a step of polymerization and/or surface cross-linking.
For example, there have been proposed various methods involving using various materials as the cross-linking agents and cross-linking conditions in the polymerization step, which include: methods involving using polyoxyalkylene glycol di(meth)acrylate or the like as an internal cross-linking agent (U.S. Pat. No. 4,351,922 and U.S. Pat. No. 5,532,323); methods involving using particular trimethylolpropane triacrylate (U.S. Pat. No. 5,574,121 and U.S. Pat. No. 5,506,324); methods involving using polyallyl ether as an internal cross-linking agent (WO 97/18890, U.S. Pat. No. 5,837,789, and WO 01/29132); methods involving using polyalkoxylated trimethylolpropane acrylic ester (WO 2003/104302, WO 2003/104299, and WO 2003/104300); and a method involving using polyunsaturated amino alcohol (U.S. Pat. No. 6,087,450).
Also proposed is a process involving using a surfactant in combination with an internal cross-linking agent in the polymerization step (U.S. Pat. No. 4,286,082, U.S. Pat. No. 5,985,944, and U.S. Pat. No. 6,251,960).
Further, there has been proposed a method involving using as cross-linking agents particular three cross-linking agents in combination (U.S. Pat. No. 5,837,789).
U.S. Pat. No. 4,654,039, U.S. Pat. No. 5,562,646, U.S. Pat. No. 4,351,922, U.S. Pat. No. 5,532,323, U.S. Pat. No. 5,574,121, U.S. Pat. No. 5,506,324, WO 97/18890, U.S. Pat. No. 5,837,789, WO 01/29132, WO 2003/104302, WO 2003/104299, WO 2003/104300, and U.S. Pat. No. 6,087,450 each propose methods involving using particular cross-linking agents and particular cross-linking conditions to improve the physical properties of the water-absorbent resins. However, those methods cause problems such as lowering the safety and surface tension due to presence of the cross-linking agents, delay of polymerization and increase in the amount of residual monomers, and coloring of final products, so it is hardly to say that the physical properties of the water-absorbent resins have been sufficiently improved. Meanwhile, methods involving using surfactants in the polymerization step, which are disclosed in U.S. Pat. No. 4,286,082, U.S. Pat. No. 5,985,944, and U.S. Pat. No. 6,251,960, sometime cause problems such as not only increasing cost but also lowering the surface tension due to the residual surfactants. In addition, the technology disclosed in U.S. Pat. No. 5,837,789 insufficiently improves the absorption capacity of the water-absorbent resin from the aspect of affinity to acrylic acid monomers and copolymerization characteristics.